In most production vehicles, the water pump that causes engine coolant to circulate through the engine and radiator is driven by the engine and the speed of the pump is dictated by the rotational speed of the engine. To ensure that there is sufficient coolant flow at the most demanding operating condition, the amount of flow at most operating conditions is higher than necessary. To improve control over the pump speed, the pump is decoupled from the engine and is either driven by an electric motor, driven by a variable speed clutch, hydraulically driven, or driven by some other actively controllable means. The electrically driven variant is particularly suited to a vehicle with a significant capacity for electrical power generation such as a hybrid electric vehicle.
It is common for a fan to be provided to direct air flow across the fins and tubes of the radiator. The fan is commonly electrically driven, although it too may be driven by a variable speed clutch, hydraulically driven, or driven by some other actively controllable means. The flow across the radiator is due to movement of the vehicle and the fan.
When an increase in heat transfer rate is indicated, the fan speed or the coolant pump speed may be increased.